1. Field of the Invention
The invention relates to the field of electrophysiology and more particularly to the sensing of extremely localized intracardiac electrical patterns in an ablating catheter.
2. Description of the Prior Art
The prior focus of electrophysiology has been directed to identifying the mechanisms of heart arrhythmias and evaluation of drug and other therapies upon the observed arrhythmias. Current studies in electrophysiology have continued to evolve by identifying localized areas of abnormal myocardium as the sources of arrhythmias and to selectively remove or otherwise deactivate the abnormal myocardium. Obliteration has generally been practiced through localized ablation, generally using a short range radio frequency diathermy technique, although other energy sources such as laser energy, ultrasound and/or cyroprecipitation may also be used.
The treatment of heart arrhythmias has thus become increasing dependent upon the ability to identify the precise anatomic location or origin in the myocardium of the abnormal rhythms. The prior art practice for locating the abnormal myocardium is to dispose a catheter within the heart chamber carrying a standard array of ring and tip electrodes. Direct contact of the tip electrode is used for making an intracardiac electrogram in a manner similar to that which has been practiced for many years with respect to pacemaker sensing. See, Imran, "Endocardial Mapping and Ablation System and Catheter Probe", U.S. Pat. No. 5,156,151 (1992).
Consider the teachings of the prior art with respect to mapping local cardiac signals, and sensing the very tissue which is being stimulated by the pacing pulse or being ablated. Goldreyer, "Method and Probe for Sensing Intracardiac Signals," Canadian Patent 1,192,263 is a 1985 patent by the applicant directed to a method and probe for sensing signals within the heart for the purposes of making EKG's and does not specifically address mapping, or ablation. Goldreyer, Canadian '263 teaches a method for sensing heart activity corresponding to a depolarization vector in the heart in terms of sensing local cardiac signals on orthogonal electrodes. The teaching is also directed to providing a ventricular pacing signal upon verification of existence of certain discriminatorily sensed local cardiac signals elsewhere in the heart, primarily in the atrium. The step of sensing does not expressly contemplate sensing from the precise area being paced; Goldreyer specifically teaches that such sensing shall be from areas removed spatially from the location of pacing. The possibility of simultaneously sensing localized cardiac activity specifically from the precise tissue where a stimulating pulse is being applied via the same catheter or sensing electrical activity from tissue while simultaneously applying ablative energy through the catheter tip to the sensed area is not implicitly or explicitly addressed nor motivated in any sense by Goldreyer, Canadian '263.
One of the reasons for lack of motivation from Goldreyer, Canadian '263 with such simultaneous steps of sensing and pacing or ablating, is that the signal strength received by the local cardiac signals is derived only from tissue near the orthogonal electrodes. The sensed area is small and the electrical signal generated by the sensed area is correspondingly weak. The very strong pacing or ablating signal energy was thought to completely swamp out the weak sensed local cardiac signal in any type of detection circuitry capable of sensing both.
In conventional catheters, such as catheters having ring electrodes, the electronics coupled to the sensing electrodes are turned off or the signal received by them is simply ignored during the time in which the pacing or ablating signal occurs. If the sensing electrodes are not electrically disconnected from the sense amplifier during the pacing or ablating phase, the sense amplifier is saturated and requires that a substantial recovery time before it can again sense signals of the magnitude generated by the local myocardium. The result is that signals during this recovery phase are simply missed.
The situation is similar in concept to radio transmissions in which the receiver and transmitter share the same antenna in a transceiver. When the transmitter is broadcasting, the receiver portion of the transceiver is disconnected from the antenna so that it is not swamped or saturated by the strong transmitting signal. Only after the transmitting signal is off, does electronic switch reconnect the radio receiver to the antenna so that the dramatically weaker received signals can be heard.
Second, even if Goldreyer, Canadian '263 is read as motivating the sensing of signals while pacing or ablating elsewhere in the heart, it must be recognized that Goldreyer, Canadian '263 actually teaches sensing in the atrium or at least a meaningful distance away from the pacing tip. None of the Figures in Goldreyer, Canadian '263 show the orthogonal sensing electrodes anywhere near the pacing tip, i.e. within a few millimeters of the pacing tip. It must be recognized that blood is an extremely lossy medium to high frequency electromagnetic waves. It is in essence sea water which is well known to be highly opaque to radio emissions. It is for this reason that it is impossible, for example, to detect submarines by radar or for submarines to communicate through radio transmissions with other stations unless an antenna is surfaced.
Therefore, even if Goldreyer, Canadian '263 can be understood to motivate simultaneous sensing and pacing, a position which the applicant firming rejects, it is by no means obvious even in hindsight that simultaneous sensing could be obtained if the orthogonal electrodes are very close the pacing or ablating tip. In Goldreyer, Canadian '263 the sensing electrodes are far enough from the pacing tip that the loss incurred in the blood between the tip and electrodes is large enough that substantial attenuation of the pacing pulse at the relatively distant electrodes might be expected. This leaves unanswered the question of whether the orthogonal electrodes could still sense local myocardial signals if they were proximate to the pacing or ablating tip. Based on Goldreyer, Canadian '263, the outcome of such an experiment could not be predicted.
Jackman et al., "New Catheter Technique for Recording Left Free-Wall Accessory Atrioventricular Pathway Activation Identification of Pathway Fiber Orientation," Circulation, Part 1, Volume 78, Number 3, September 1988, describes an application of an orthogonal sensor in the coronary sinus on the exterior of the surface of the heart. Orthogonal sensors A, B and C are shown in FIG. 1 have large scale separations of 10 millimeters. As stated in the right column at page 599, the catheter was advanced into the coronary sinus and positioned as anteriorly as possible in the great cardiac vein. Electrograms from the electrodes were examined to identify which electrodes faced the myocardium. Certain ones of electrodes, i.e. the half rings, were then identified as being closest to the heart tissue.
All sensing described by Jackman was conducted solely in the coronary sinus and great cardiac vein. The catheter was never placed within the heart and it does not appear that the use of the orthogonal probes in the catheter within the heart occurred to Jackman.
Further, it does not appear that Jackman conducted either any pacing or ablation while attempting to simultaneously sense local myocardial signals with the orthogonal electrodes. In fact, the catheter shown in Jackman contained no tip electrode capable of either pacing or the application of ablative energy. The purpose of the Jackman study was an attempt to locate and trace the Kent bundles and accessory pathways on the outside of the heart and not to discriminatorily sense or pinpoint malfunctioning myocardial tissue inside the heart. Being able to sense electrical activity on the outside of the heart wall is a prior art technique which does not include, contemplate or motivate myocardial mapping or selective ablation deep in the interior heart tissues. Nor does this technique teach localized sensing during the application of ablative energy.
Jackman describes the orthogonally spaced electrodes, but does not explicitly describe any differential amplification of the signals from the electrode pairs, nor local sensing of the heart signals.
Furthermore, the orthogonal electrodes are actually split rings and not dot or microelectrodes. Jackman does not monitor a differential signal between the ring electrode halves, but examines "the electrode facing the myocardium," namely that electrode or portion of the split ring facing the myocardium. There is no motivation, suggestion or teaching in Jackman, which would suggest that closely spaced bipolar differentially amplified electrodes can be used for mapping local myocardial signals inside the heart or simultaneously used for sensing during pacing or ablation.
Hess et al., "Percutaneous Lead Having Radially Adjustable Electrode," U.S. Pat. No. 4,660,571 (1987) describes a lead which is provided for the endocardial functions of mapping, ablation and/or pacing. Elongated electrodes on the distal end of the catheter are moved radially outward to provide a plurality of radially adjustable contact electrodes for the purposes of mapping. An electrode is provided on the tip also for ablation and pacing. Hess shows what is a conventional spring or spider array of contact electrodes in FIGS. 3 and 4 which are disposed outwardly to physically contact the heart wall. Not only is the position of these electrodes difficult to control, but they also tend to be spaced apart by a minimum predetermined distance as determined by the extent of radial splay of the flexible fingers 32 from the end of the catheter shown in FIG. 1 in the nondeployed configuration and in the deployed configuration in FIGS. 3 and 4.
What Hess describes, in particular beginning at column 6, lines 15-60, is a mapping or positioning of the electrode within the heart through the use of the contact electrodes and thereafter ablating the endocardial site where the abnormal focus has been located. There is no suggestion, teaching, motivation or claim made by Hess that he is able to continue sensing the local myocardial site which is being subject to ablation. Instead, Hess teaches a two-step process in which the abnormal focus is located, and once located then ablated. Hess does not employ orthogonal electrodes, nor describes, nor addresses the problem of how to avoid saturating his sensing electronics during the strong radiofrequency ablation. This was not even a problem which concerned Hess since he did not use radio frequency energy to ablate, but used a laser power source through an optical fiber as the ablating element. The suggestion at column 7, lines 1-14, that sensing and pacing may also be performed, is not described as being simultaneous, does not address the problem of sense amplifier saturation, and does not describe differential amplification of the electrode signals. Hess' suggestion that control of dysrythmia by pacing the ventricle merely suggests the use of the tip electrode 37 and ring electrode 47 as a bipolar electrode pair and does not address how the pacing electrodes can be simultaneously used with other sensing electrodes and in close proximity to each other so that the localized myocardium, which is being paced, can be simultaneously sensed. There is nothing in Hess which suggests that this could even be done. It was the conventional wisdom at the time of Hess, in fact, that this could not be done and it has only been shown on the basis of applicant's teaching that it is possible.
One of the recognized problems in prior art pacemaking has been the ability to simultaneously monitor the activity within the heart chamber while a large ventricular stimulating pulse was delivered through the catheter tip. One prior art solution is shown in Goldreyer, "Catheter, Cardiac Pacemaker and Method of Pacing", U.S. Pat. No. 4,365,639 (1982), wherein orthogonal sensing electrodes positioned on the catheter in the atrium were able to sense heart activity without being overwhelmed or saturated by the large ventricular stimulating pulse delivered through the catheter tip. In other words, because of the orthogonal placement of the sensing electrodes within the catheter body relative to the stimulating tip and the differential signal processing from the orthogonal electrodes, signals in the heart from directions other than the tip of the catheter could be preferentially sensed shortly after the large pacing and responsive ventricular pulse without saturation of the sensing circuitry. However, even in this prior art technology it was believed necessary to distance the sensing electrodes from the pacing tip by what is considered in terms of the present invention to be a large distance, namely 10 to 16 cm. See column 4, lines 41-42. It was not known or understood that the very tissue being paced could also be simultaneously sensed. What was being sensed was tissue far away from the site of stimulation, 10-16 cm away, which was not responding to the pacing spike, but to the heart propagation wave.
Electrophysiology has gone from an era where the purpose was the evaluation of the mechanisms or arrhythmias and evaluation of drug therapy to one which involves the localization of areas of abnormal myocardium in order to remove or obliterate them. In this regard, the mapping of the precise origin of the abnormal rhythms is extremely important. In the prior art, standard ring electrodes are placed on the catheter and the catheter moved within the cardiac chambers in an attempt to find the tissue spot which depolarized first and was the focus of the arrhythmia. Such probes have been of some use to spatially identify points in the myocardium having particular electrophysiological activity, however, signals derived from this standard electrode array suffer from considerable far-field influences and lack precise localization for accurate mapping purposes.
In clinical electrophysiology, recent studies have stressed that the localization of the site from which abnormal rhythms originate may be confirmed by pacing from the mapped area and demonstrating that the surface activation is identical to that seen during the spontaneous arrhythmia. Using standard ring electrodes as is currently done, intracardiac electrograms cannot be simultaneously recorded from the site of stimulation during these pacing sequences. This can be achieved by the invention.
What is needed is a method and apparatus for reliably mapping discrete electrophysiologic activity in the heart without the need to contact the heart walls and which sensing from the localized myocardium can be done simultaneously with pacing or ablation procedures from the precise tissue being stimulated.